By Pete Singer, Editor-in-Chief
The move to electric vehicles (EVs), combined with new legislation focused on renewable energies, has led to a greatly increased demand for power electronics based on silicon carbide (SiC). “There’s a lot of new demand for silicon carbide power devices to build the infrastructure for renewable energy power distribution, and for charging stations for electric cars,” said Martin Tollner, President Semiconductor Chamber Solutions at Edwards Vacuum.
According to Wolfspeed, when compared to their Si counterparts, SiC MOSFETs offer better overall performance, higher efficiency, higher switching frequencies, and more compact components. SiC‘s advantages include:
- A higher critical breakdown field, which means a voltage rating can be maintained while still reducing the thickness of the device
- A wider bandgap, leading to lower leakage current at relatively high temperatures
- A higher thermal conductivity, which supports a higher current density
- An overall reduction in energy losses
Using SiC in place of Si in MOSFETs also results in:
- Reduced switching losses, which impact losses that occur when the MOSFET is transitioning from blocking to conducting (and vice versa)
- Higher switching frequencies, which means smaller peripheral components (e.g., filters, inductors, capacitors, transformers) can be used
- Increased critical breakdown strength, about 10x what is achiev- able with Si
- Higher temperature operation, which means simplified cooling mechanisms (e.g., heat sinks)
For EVs, SiC power semicon- ductors help efficiently convert battery power to torque, thereby increasing vehicle performance and range.
Processing changes: Hot implant and larger wafers
Compared to silicon, SiC is inherently harder with natural defects that can lead to degradation of electrical performance, power efficiency, reli- ability and yield. Advanced materials engineering is needed to optimize raw wafers for production and build circuits with minimum damage to the crystal lattice.
During SiC chip fabrication, ion implantation places dopants within the material to help enable and direct the flow of current within the high power producing circuits. The density and hardness of SiC material makes it extremely challenging to inject, accurately place and activate the dopants while minimizing damage to the crystal lattice which reduces per- formance and power efficiency. New “hot implant” technology gets around this by heating the wafers. Applied Materials new VIISta® 900 3D hot ion implant system, for example, injects ions with minimal damage to the lattice structure, resulting in a more than 40X reduction in resistivity compared to implant at room temperature. The tool was introduced late last year, in September.
SiC devices are also moving to larger wafer sizes in order to increase throughput. Chipmakers are transitioning from 150mm wafer production to 200mm production, which approximately doubles die output per wafer.
Vacuum pumping challenges and a new solution
These two trends – larger wafers running at hotter temperatures – has created some unique new challenges when it comes to vacuum pumping of the ion implant chamber. Not only do larger amounts of hydrogen have to be pumped, it has to be done in a higher temperature environment. “You can imagine a wafer being heated to 500°C, and a cryogenic pump trying to pump hydrogen — which has to be done at extremely low temperatures — operating in the same environment,” Tollner said. “It does lend itself to some really big challenges.”
Enter the new Edwards XVS (extreme vacuum stability) cryopump. “We developed the XVS product specifically around those challenges, which was how do we provide a product, which is phys- ically the same size as the older product, but provides considerably more hydrogen pumping speed,” Tollner said.
Within the CTI-Cryogenics On-Board IS 320F XVS Cryopump is an intelligent system control integrated in the cryopump ensures better process quality, vacuum consistency, and uptime, while providing real-time system infor- mation for optimum control of array temperatures. Vacuum quality is enhanced by automatic adaptation to changing thermal/gas loading conditions.
“Over time, you don’t want to see that pumping speed degrade through contamination,” Tollner said. “You want to try to extend it as long as possible. We’ve developed some technology within the array design, which allows us to have that lower variation or loss of pumping speed due to contamination. We can really extend those service intervals and the life of the product, and also ensure that the pumping speed variation is very small from cycle to cycle.”
Edwards has also improved the sustainability aspect of the new cryopump by developing new compressor technology.
“Typically we have three cryo- genic pumps on an implant chamber with a very large helium compressor to help generate the helium around the system. Those compressors do tend to consume
quite a lot of power,” Tollner said. “Part of the XVS product range was also to develop a new compressor technology, which reduces the power by about 20%, which makes the entire package in terms of cost of ownership more effective for the customer.”